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DECEMBER 2020 I DESIGN007 MAGAZINE 65 by Dugan Karnazes VELOCITY RESEARCH I went into the field of engineering to solve problems and come up with practical solutions. But every now and then, a solution comes up that is today unbuildable. to understand for determining the proper Dk value to use for circuit design and simulation. There is a complication in which the cir- cuit design and construction can be more or less sensitive to the copper surface roughness effects. The microstrip circuit example given is relatively easy to understand for the effects of copper surface roughness. However, for a strip- line circuit, which has four substrate-copper interfaces, the influence of surface roughness on circuit performance is more complicated. The stripline circuit can have very different surface roughness effects due to how the cir- cuit is constructed. A stripline circuit fabri- cated using a foil lam construction can have a very different copper roughness influence than a stripline made as a core lam construction. There is also the potential influence of some PCB fabrication processing effects on the extracted Dk of a material when tested in circuit form. One of these influences can be the cop- per plating thickness and its normal variation. Another influence can be the final plated fin- ish and its variation. The copper plated thick- ness and its variation, as well as the final plated finish and its variation, can have a significant impact on circuit features that are edge coupled. These potential influences on circuit per- formance, which can affect the extracted Dk value of the material, will depend a lot on the circuit design and construction. Most design- ers consider the final plated finish in relation to insertion loss differences; however, the plated finish can certainly impact the phase response of the circuit, which can alter the extracted Dk value of the material from a cir- cuit evaluation. The bottom line for getting good results for a first-time evaluation of a new circuit design is for the designer to understand the material properties, test methods, and influences of circuit fabrication for their particular design concept. These are a lot of issues to consider, and it is best to have the designer work closely with their material supplier and the PCB fab- ricator to ensure the best results for the initial circuit evaluation of a new design. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here. I would like to introduce you to a problem that has been bouncing around in my head: the field-programmable bat- tery array. One of the notable design challenges with this is the limitations of today's transistors. I fully acknowledge that this design is impractical today, but with better transis- tors (or a better designer), that could change. As history shows us, anyone who has bet against the advance- ment of transistors has been disappointed, so I hold hope that one day, this idea will be practical. This is more of a thought experiment, but perhaps one of you can get fur- ther with it than I have. To read the full column, click here. Dugan Karnazes is the founder and CEO of Velocity Research. Figure It Out: Field-Programmable Battery Array (Unsolved)

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